1. Field of the Invention
The invention relates in general to a light emitted diode driving apparatus, and more particularly to a light emitted diode driving apparatus having a constant-current circuit.
2. Description of the Related Art
A liquid crystal display uses a backlight module as a light source for a liquid crystal panel. The light source can be light emitted diodes (LEDs). The LED radiates light as a current flows by, and the LED luminance is directly proportional to the amount of the flowing current.
In order to provide the required LCD luminance, the backlight module has to use several LEDs for generating enough luminance. Referring to FIG. 1, a block diagram of a conventional LED driving apparatus is shown. In FIG. 1, A DC-to-DC converter 102 outputs a direct-current voltage DC to drive two LED series S1 and S2 and receives a feedback voltage Vf from the LED series S1. The DC-to-DC converter 102 adjusts the amount of voltage DC according to the feedback voltage Vf so that the LED series S1 and S2 can generate the required luminance.
However, each LED has a different cut-in voltage, so the currents Ia and Ib respectively flowing through the LED series S1 and S2 connected in parallel are different. As a result, LED series S1 and S2 have different luminance and thus the whole backlight module generates luminance of non-uniformity. Moreover, due to variation of each LED's cut-in voltage, each of the LED series S1 and S2 has a different voltage drop as in a turn-on state. Consequently, the DC-to-DC converter 102 cannot precisely control the LED series S1 and S2 to generate the same luminance according to the feedback voltage Vf.
Therefore, how to control the LED series to generate uniform luminance is a vital subject for a LED designer or manufacturer.